a) Field of the Invention
The present invention relates to a semiconductor device and its manufacture method, and more particularly to a semiconductor device having capacitors and its manufacture method.
b) Description of the Related Art
Still higher integration of semiconductor integrated circuits is required nowadays. Semiconductor memory devices with capacitors are required to form each capacitor having a three-dimensional structure, along with the miniaturization of the memory cell.
A capacitor is considered as a combination of a lower electrode, a dielectric film and an upper electrode. A capacitor of a three-dimensional structure has the lower electrode of a three-dimensional structure on the surface of which the dielectric film and upper electrode are formed. In order to reduce the capacitor electrode area and have a sufficient capacitance, it is desired that the capacitor dielectric film is made of material having a high dielectric constant, such as tantalum oxide (having a stoichiometric composition of Ta2O5, hereinafter simply written as TaO). A high dielectric constant is intended to mean a specific dielectric constant of 20 or higher.
If a capacitor dielectric film is made of ferroelectric material, a non-volatile memory can be formed which retains its memory state even if the power is turned off. Ferroelectric material may be strontium titanate (SrTiO3, hereinafter simply written as STO), barium strontium titanate (BaxSr1−xTiO3, hereinafter simply written as BST), lead zirconium titanate (Pb1−xZrxTiO3, hereinafter simply written as PZT) or the like.
These dielectric materials are oxide. It is desired to perform a heat treatment (annealing) in an oxygen-containing oxidizing atmosphere after a dielectric film is formed. Therefore, the lower electrode is desired to be made of metal having a high resistance against oxidation, metal retaining conductivity even after it is oxidized, or oxide of this metal. Rare metal such as Ru, Ir and Pt has been studied as such metal of the lower electrode. The concept of rare metal is inclusive of noble metal.
There are some problems in using rare metal as the material of electrodes and wiring lines. If rare metal is in direct contact with silicon, silicide is formed when heated. The resistivity of silicide is higher than that of metal, and the silicidation process does not occur uniformly along the depth direction (making a more irregular surface). In order not to form silicide, it is desired to insert a barrier metal layer between rare metal and silicon. As barrier metal, nitride of a single metal such as TiN and WN (representing WNx), nitride of two or more metals such as TiAlN have been studied.
Rare metal has the property of allowing passage of oxygen, hydrogen and the like. If there is a metal layer made of general wiring material such as Al, Cu and W under a rare metal layer and the underlying metal layer is oxidized by oxygen penetrated through the rare metal layer, the metal layer raises its resistivity, becomes an insulating layer, expands its volume, and so on.
A W plug is used as a conductive plug passing through an inter-level insulating film. On this W plug, a Ru layer is deposited to some thickness, for example, by sputtering, and thereafter Ru is deposited by chemical vapor deposition (CVD) by using oxygen catalyst. In this case, if the W plug is oxidized, the W plug and upper structure may be broken by volume expansion of the plug.
A capacitor is constituted of a pair of electrodes and a dielectric film between the electrodes. If the electrodes are flat, the electric field between the electrodes is generally uniform. However, if the electrode has a three-dimensional structure, the electric field generated in the dielectric film is not necessarily uniform.
A cylinder type capacitor has a lower electrode of a cap shape. The top surface of the lower electrode is defined by chemical mechanical polishing (CMP) or the like, and is generally perpendicular to its side walls. An electric field concentrates upon the boundary between the top surface and side wall of the lower electrode. If the dielectric film has a uniform thickness, the dielectric film at the boundary between the top surface and side wall becomes more likely to be dielectrically broken. If the dielectric film is made thick in order to avoid such dielectric breakdown, the dielectric film becomes unnecessarily thick on the flat surface area.
As a Ru layer is patterned by using a resist mask, etching by-products called a fence are deposited on the resist mask. The by-products once formed are difficult to be removed.
Generally, transistors are annealed in a hydrogen atmosphere in order to improve their characteristics. Ru is considered to generate hydrogen radicals through catalyst reaction during this annealing. The dielectric oxide film of a capacitor may be reduced by the hydrogen radicals, so that its characteristics may change.
In the above description, although Ru is mainly used as the electrode material, similar problems may occur even if other rare metals are used.
As described above, various problems to be solved occur as the structure is made smaller.